Linear beam electron devices are used in sophisticated communication and radar systems that require amplification of a radio frequency (RF) or microwave electromagnetic signal. A conventional klystron is an example of a linear beam electron device used as a microwave amplifier. In a klystron, an electron beam is formed by an electron gun which has a thermionic cathode with a negative pulsed or direct current (DC) voltage which thermionically emits electrons that are attracted to a grounded anode through a focusing electrode which shapes the electron field. The comparatively positive voltage of the anode accelerates the thermionically emitted electrons, with the electron beam confined to travel in a beam tunnel through the application of an external axial magnetic field. The electrons originating at the cathode of the electron gun propagate through a drift tube comprising an equipotential surface which encompasses the electron beam tunnel, thereby eliminating the accelerating force of the applied voltage. The drift tube includes a number of gaps that define resonant cavities of the klystron. The electron beam is velocity modulated by an RF input signal introduced into one of the resonant cavities. The velocity modulation of the electron beam results in electron bunching due to electrons that have had their velocity increased, gradually overtaking those that have been slowed. The traveling electron bunches represent an RF current in the electron beam, and this RF current induces electromagnetic energy into a subsequent one of the resonant cavities positioned along the beam tunnel. The electromagnetic energy may then be extracted from a subsequent resonant cavity as an amplified RF output signal.
Since the invention of the klystron, it has been recognized that a klystron having multiple electron beams, each beam travelling in a separate drift tube, would have certain advantages over a klystron having a single electron beam in a single drift tube. There are three principle advantages of a multiple-beam approach over a single beam approach. The first advantage over a single beam is the improved strength and uniformity of the electric field across the resonant cavities formed by the gaps at the ends of the multiple drift tube bundles, which are aligned about the resonant cavities, compared to the fields across the resonant cavities formed by a gap of a single drift tube. The second advantage is that electrons in one of the drift tubes are isolated from electrons in the other drift tubes. This isolation results in lower debunching, or space charge forces, since the self-repelling space charge force of electrons in the beam is increased with the greater electron beam density required by higher power devices. The reduced space charge effect in a multiple beam klystron results in a higher current, lower voltage device which typically results in a higher efficiency and higher power device compared to a conventional single beam klystron having a low current electron beam operating at a much higher voltage. The third advantage is that a multiple beam klystron can achieve much more bandwidth than a conventional klystron because the fringing capacitance and electric field around the bundle of drift tubes at each gap is a smaller fraction of the useful electric field in the gap which interacts with the electrons. The reduction factor of this capacitance is approximately equal to the number of parallel beams.